http://www.dailywritingtips.com/some-thoughts-on-dashes/
From the Blog:
A reader wonders about the use of dashes to replace colons:
More and more I see dashes (of various sorts) used to do the work you say that the colon should do: introduce a word, phrase, clause, list, or quotation after a complete sentence. Is either acceptable?
I can’t find an example of a dash used to introduce a quotation, but its use in place of a colon to introduce a word, phrase, clause, or list is common:
“Nonetheless,” he added, “just having these recordings is not going to be sufficient” to make any definitive conclusions about the cause of the crash—a process that could take weeks, if not months.”
Now students’ needs are anticipated by a small army of service professionals—mental health counselors, student-life deans and the like.
Coming at the end of a sentence in this way, the use of a dash is not as jarring to me as its increasingly popular use to replace commas or parentheses within a sentence:
The study’s authors hypothesized that material gains made through early agricultural success—a proxy for wealth—gave smaller groups of related men the reproductive upper hand for generations.
Boko Haram has widened its efforts from capturing foreigners—who can be ransomed off for big bucks—to targeting mass numbers of young women and children who can be put to other uses.
Commas or parentheses would do just fine in each example.
The choice to replace commas or parentheses with a dash should be made with a clear understanding of the effect desired.
Explanatory information meant for readers who may need help with a concept can go in parenthesis:
The study’s authors hypothesized that material gains made through early agricultural success (a proxy for wealth) gave smaller groups of related men the reproductive upper hand for generations.
Information relevant to the sentence, but of secondary importance can go between commas:
Boko Haram has widened its efforts from capturing foreigners, who can be ransomed off for big bucks, to targeting mass numbers of young women and children who can be put to other uses.
Dashes are appropriate when the purpose is to startle the reader with an unexpected interruption that provides a peripheral thought:
His chisel was one of the weapons used—not that he could help that, poor fellow—and no doubt you will want to ask him questions.
The dash is an attention-getting punctuation mark that can be used to change the tone of a sentence. Dashes are like the unexpected chords in The Surprise Symphony: they jerk the reader into wakefulness. They are most effective when not overused.
